1. Field of the Invention
The present invention relates to a line coupling structure for coupling a dielectric waveguide, in which signals such as millimeter waves propagate, to a suspended line. The present invention also relates to a mixer including such a line coupling structure and further relates to a receiving/transmitting apparatus including such a mixer.
2. Description of the Related Art
A diode mounting structure in a nonradiative dielectric waveguide (hereinafter referred to as an NRD waveguide) and a mixer with such a structure are disclosed in Kuroki and Yoneyama “Circuit Elements In Nonradiative Dielectric Waveguide Using Beam Lead Diodes”, Journal of IEICE (The Institute of Electronics, Information and Communication Engineers), C-I, Vol J-73-C-I, No. 2, pp. 71-76 (February 1989).
This mixer has a structure in which a coupler includes an NRD waveguide, and a circuit board carrying a diode is vertically sandwiched between dielectric strips to couple the diode to the NRD waveguide.
However, the structure disclosed in the above-described document has various problems. Specifically, since the circuit board carrying the diode is arranged in a direction that is perpendicular to the lengthwise direction of the dielectric strips, it is difficult to have the circuit board be fixed and it tends to tilt, which makes it difficult to mount. Insertion of a sheet having a high dielectric constant into the NRD waveguide, providing a gap therein, or other measures are required to achieve matching in the structure, and therefore, the above-described structure cannot be easily designed and fabricated. In a coupler including the NRD waveguide, the greater the difference from the frequency at which the power distribution ratio is even, the higher the possibility that the power distribution ratio lacks balance.
In Japanese Unexamined Patent Application Publication No. 10-75109, a mixer having a line coupling structure for coupling a dielectric waveguide to a suspended line is disclosed. A typical mixer disclosed in the above-described publication is shown in FIG. 6. FIG. 6 is a plan view showing the dielectric waveguide apparatus when an upper conductor plate is removed. A circuit board 4 and dielectric strips are sandwiched between two conductor plates including two parallel conductor planes (not shown in FIG. 6). A dielectric strip 3b in FIG. 6 is an upper dielectric strip disposed on the circuit board 4. Another dielectric strip facing the dielectric strip 3b is disposed beneath the circuit board 4. On the circuit board 4, a conductor pattern 5 having open stubs 6a, 6b, 7a, and 7b, each having a length of about λ/4, is provided. A beam lead diode 8 is mounted on and connected in series with the conductor pattern 5. The dielectric strip 3b is arranged such that it crosses the conductor pattern 5 in a direction that is perpendicular to the conductor pattern 5 at a predetermined distance from the inner end thereof. The line coupling structure for coupling the suspended line including the conductor pattern 5 and the upper and lower conductor plates to a NRD waveguide including the dielectric strip 3b and the upper and lower conductor plates is formed in such a manner.
Since the dielectric waveguide apparatus described above has a structure in which LSM mode signals propagating in the dielectric waveguide have the same magnetic field direction as TEM mode signals propagating in the suspended line, the waveguide is easily and strongly coupled to the suspended line. Accordingly, this apparatus has various advantages including conversion loss in the mixer which can be less than that in known apparatuses, and the simplified structure of the overall apparatus allows for easy manufacturing.
However, the inventors of the present invention have discovered by experiment and determined that the transmission loss in a line coupling section between the dielectric waveguide and the suspended line can be further reduced.